CorpusHuman Anatomy
MuscularMolecule

Thick filament (myosin)

Myofilamentum crassum

Also known as: myosin, myosin II, myosin filament, cross-bridge, myosin head

A bipolar filament of ≈300 myosin II molecules (≈1.6 µm long) whose heads form cross-bridges with actin and hydrolyse ATP to generate force.

Sarcomere & Sliding Filaments

Overview

Length≈1.6 µm
Myosin molecules per filament≈300
Power-stroke step≈5–10 nm per ATP

Each myosin II molecule is a hexamer of two heavy chains and four light chains (two essential and two regulatory). The heavy chains coil together into a long α-helical tail. At one end each heavy chain forms a globular head (subfragment-1). The head contains an actin-binding site and an ATPase site, and a neck/lever arm stabilised by the light chains. Tails pack together to form the filament backbone, with heads projecting toward each Z disc. Near the M line there is a central bare zone without heads, because the molecules are arranged in opposite directions in each half. This bipolar arrangement lets both halves of the sarcomere pull toward the centre.

The heads project in a helix around the filament, so each thick filament can interact with its six surrounding thin filaments. In each cross-bridge cycle, ATP binding releases the head from actin. Hydrolysis to ADP + Pᵢ re-cocks the head. Binding to actin and releasing Pᵢ triggers the power stroke, a ≈5–10 nm swing of the lever arm. Myosin-binding protein C and titin regulate and organise the filament. The rate of the myosin ATPase sets the speed of shortening and differs between fibre types (MHC-I, IIa, IIx).

Parts & subdivisions

  1. Myosin head (S1)
    Contains the actin-binding and ATPase sites. It forms the cross-bridge.
  2. Lever arm (neck)
    An α-helix stabilised by the essential and regulatory light chains. It amplifies small changes in the head into the power stroke.
  3. Tail (rod)
    Coiled-coil heavy chains that assemble into the filament backbone.
  4. Bare zone
    The central region without heads, where tails meet in antiparallel orientation (M line region).

Function

  • Generates force and movement by cyclically binding actin
  • Hydrolyses ATP (myosin ATPase)
  • Sets contraction speed through its heavy-chain isoform

Clinical significance

Hypertrophic cardiomyopathy

Mutations in β-myosin heavy chain (MYH7) or myosin-binding protein C (MYBPC3) are the commonest causes of hypertrophic cardiomyopathy, a leading cause of sudden cardiac death in young athletes.

Test yourself

Without ATP, at which point does the cross-bridge cycle stop?
  1. Myosin cannot bind actin
  2. Myosin stays bound to actin after the power stroke (rigor)
  3. Ca²⁺ cannot bind troponin C
  4. Tropomyosin cannot cover the binding sites

Answer: Myosin stays bound to actin after the power stroke (rigor)

ATP binding is what detaches the myosin head from actin. Without ATP the heads remain locked in the rigor state, which is the basis of rigor mortis.

In smooth muscle, which enzyme is activated by Ca²⁺–calmodulin to start contraction?
  1. Troponin C
  2. Myosin light-chain kinase
  3. Myosin light-chain phosphatase
  4. Creatine kinase

Answer: Myosin light-chain kinase

Smooth muscle has no troponin. Ca²⁺–calmodulin activates MLCK, which phosphorylates the regulatory light chain of myosin so cross-bridges can cycle. MLC phosphatase causes relaxation.

More muscular system questions

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.